In reality I could just hang a water alarm from Lowe's in the tank at the height I want it to alert me. 85dB of loud will get my attention....but automatic shut off...just is a cool concept. I think out of the box guys..that's what built our bus...thinking out of the box...so I'll keep scratching my head on this one. Thanks...

That's basically what I'm looking for...it would work...so would the toilet fill kit..if I could retrofit, but remember, we're talking about filling a 100 gallon tank..at 1/4" it would in theory take twice as long as the 1/2" line I'm currently using...right? I just thought someone out in the RV world came up with this already...

A 1/4" line will take you A LOT more than twice as long to fill than a 1/2" line! Don't even think about filling a 100 gallon tank through that - it's not much larger than a drinking straw! FYI, it takes me at least ten minutes to fill either of my 110 gallon tanks from 65 PSI city water through a 5/8" hose, using either a 3/4" direct tank feed or a 1-1/4" gravity fill.

Water connection is in the bottom, water comes out around the top. I would add support at the top. Install in tank if possible or in a 4in PVC pipe that is tied to the tank at the top and bottom so that the water level in the pipe is the same as in the tank. You might have to throttle the water flow because the fill water will have to flow through the bottom connecting pipe into the tank. You still need an overflow.

Another way is to salvage the pressure switch and solenoid from an old washing machine. Tie the pressure switch pressure tube to the tank at the bottom with the switch located so that it opens at the high water level. Break the 120 VAC going to the solenoid through the float switch. These solenoids even come with a hose connection.

... we're talking about filling a 100 gallon tank..at 1/4" it would in theory take twice as long as the 1/2" line I'm currently using...right? ...

No. A 1/2" line is FOUR times as large as a 1/4" line -- remember that cross-sectional area of a circle varies with the square of the diameter. So at the very best, it would take four times as long. In practice, it takes even longer, because as the diameter decreases, the ratio of the surface area of the pipe to the volume of the pipe also increases, and flow rate at a given pressure will be lower. That calculation depends on the material of the pipe, number of bends, etc., and is more complex than we need to go into, other than to say you should figure it takes more like five times as long to deliver a give quantity of fluid through a pipe half the diameter.

To elaborate on what John wrote, at most city water fills, it takes us 20 minutes to fill our 135-gallon tank through a 5/8" hose (the smallest diameter in the system). If we added a 1/4" restriction to that line I would figure it to take more like an hour and a half, up to two hours.

This is one reason why I tell folks to make sure they understand orifice diameters when they are buying valves. Lots of solenoids on the market, for example, will have 1/2" inlet and outlet fittings, but a much smaller orifice, some as small as 1/8".

Cool! Never did the math. But yes, it fills pretty quick on my 1/2" line, but took forever on my previous 1/4" line. So this is why. Going to look into the toilet valve again as well as the washing machine valve.